dma: amba-pl08x: use vchan_dma_desc_free_list

vchan_dma_desc_free_list() iterates through each virt_dma_desc in the
specified list_head and calls vchan->desc_free().

We can use it instead of repeated execution of pl08x_desc_free() for each
virt_dma_desc in the list_head.  Because vchan->desc_free callback is set
as pl08x_desc_free() for amba-pl08x driver.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Dan Williams <djbw@fb.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>